1. Field of the Invention
The present invention relates to a laser device and a controlling method therefor.
2. Description of the Related Art
There is known an external resonator type laser device provided with a resonator including a wavelength selective filter and a semiconductor light amplifier so as to emit a single wavelength laser beam.
FIG. 7 is a block diagram illustrating a structure of a wavelength tunable laser module 90 that is a type of the external resonator type laser device. As illustrated in FIG. 7, the wavelength tunable laser module 90 includes: a resonator 20 including a mirror 22, a wavelength tunable filter 24 (first wavelength selective filter), an ITU grid filter 26 (second wavelength selective filter), a collimator lens 28, and a semiconductor light amplifier 30; a collimator lens 36; a beam splitter 38; a condenser lens 40; and an optical output detector 42, so as to deliver a single wavelength laser beam oscillated in the resonator 20 to an optical fiber 44.
In FIG. 7, the collimator lens 28 disposed at a left side of the semiconductor light amplifier 30 converts light emitted from an end surface 31 of the semiconductor light amplifier 30 into collimated light.
The mirror 22, the wavelength tunable filter 24 and the ITU grid filter 26 are disposed on an optical axis of the collimated light converted by the collimator lens 28. The mirror 22 is disposed to be perpendicular to the optical axis of the collimated light.
The semiconductor light amplifier 30 includes a light amplifying region 34 working as a gain medium and has a structure substantially similar to a semiconductor laser except that a reflectance of the end surface 31 is so small that laser oscillation is not induced by itself compared with the semiconductor laser. Instead, the semiconductor light amplifier 30 induces laser oscillation by using a resonator 20 including a reflector (mirror 22) and another end surface 35. In other words, the light emitted from the end surface 31 of the semiconductor light amplifier 30 passes through the collimator lens 28, the ITU grid filter 26 and the wavelength tunable filter 24, and is reflected by the mirror 22. Then, the light passes through the wavelength tunable filter 24, the ITU grid filter 26 and the collimator lens 28, and enters the semiconductor light amplifier 30. The light that has entered the semiconductor light amplifier 30 is amplified by the light amplifying region 34 and is reflected by the end surface 35 (a part of the light passes through the same) so that the light is emitted from the end surface 31 again. Note that the laser oscillation is induced when a gain (light amplification factor) of the semiconductor light amplifier 30 is larger than a loss in the resonator 20.
A wavelength of the laser beam can be selected in a variable manner by the wavelength selective filter (in particular, the wavelength tunable filter 24) which is capable of changing an operating wavelength and is disposed between the mirror 22 and the end surface 31 of the semiconductor light amplifier 30.
The semiconductor light amplifier 30 further includes a phase adjustment region 32 having a variable refractive index that varies in accordance with a signal supplied from the outside (that is usually current, and is hereinafter referred to as “phase adjustment signal”). When the refractive index of the phase adjustment region 32 varies, a phase of the light passing through a phase adjustment region 32 changes. As a result, a vacuum conversion length (hereinafter referred to as “effective length”) of the resonator 20 varies. In other words, according to the semiconductor light amplifier 30, the effective length of the resonator 20 can be adjusted by changing the phase adjustment signal to be applied to the phase adjustment region 32. Note that JP 3-129890 A discloses a light source device having a single semiconductor element including a phase adjustment region and a light amplifying region so as to control the two regions individually.
In FIG. 7, a structure for monitoring intensity of the laser beam emitted from the end surface 35 of the semiconductor light amplifier 30 and a structure for delivering the laser beam to the outside are illustrated on a right side of the resonator 20.
The collimator lens 36 disposed at a right side of the semiconductor light amplifier 30 converts the laser beam emitted from the semiconductor light amplifier 30 to the collimated light.
The beam splitter 38 branches a part of the collimated light converted in the collimator lens 36 in a direction of the optical output detector 42. The optical output detector 42 monitors the part of the collimated light branched by the beam splitter 38, and detects the light intensity thereof.
The condenser lens 40 condenses the collimated light that has passed through the beam splitter 38 and delivers the condensed light to the optical fiber 44 as a laser beam. Thus, the laser beam oscillated in the wavelength tunable laser module 90 is taken out as an optical output to the outside.
Next, a laser oscillation wavelength (hereinafter simply referred to as “oscillation wavelength”) of the wavelength tunable laser module 90 is described. As to the wavelength tunable laser module 90, there are three factors that determine the oscillation wavelength. The factors include a wavelength of light oscillated in the resonator 20 illustrated in FIG. 1(a) (hereinafter referred to as “resonance wavelength”), light transmission characteristics of the wavelength tunable filter 24 illustrated in FIG. 1(b), and light transmission characteristics of the ITU grid filter 26 illustrated in FIG. 1(c).
The resonance wavelength of the resonator 20 is a wavelength that forms a standing wave having nodes at the mirror 22 and the end surface 35 of the semiconductor light amplifier 30 in the resonator 20. Specifically, the resonance wavelength is a wavelength satisfying a condition “the length of the resonator 20 is an integral multiple of a half of the resonance wavelength (in the case where the refractive index in the resonator is 1)” (hereinafter referred to as “resonance condition”). According to this resonance condition, the resonance wavelength depends on the length of the resonator 20, and is distributed in a discrete manner within a wavelength band as illustrated in FIG. 1(a) (for instance, if the vacuum conversion length of the resonator is 15 mm and the oscillation wavelength is 1,550 nm, a resonance wavelength interval is approximately 80 pm). Note that a wavelength mode that satisfies the resonance condition of the resonator 20 is referred to as a resonator mode (cavity mode).
The oscillation wavelength of the wavelength tunable laser module 90 is a wavelength that can exist stably in the resonator 20 among a plurality of resonance wavelengths that satisfy the resonance condition described above. Specifically, the oscillation wavelength is the resonance wavelength of light that passes through the two wavelength selective filters (wavelength tunable filter 24 and ITU grid filter 26) disposed between the mirror 22 and the collimator lens 28.
The wavelength tunable filter 24 is a wavelength selective filter having characteristics in which only the light belonging in a specific wavelength band is permitted to pass through while other light is reflected, scattered or absorbed. As illustrated in FIG. 1(b), the wavelength tunable filter 24 usually has one peak transmission wavelength (wavelength at which a transmittance is peak) in an operating wavelength band of the wavelength tunable laser module 90, and a transmission wavelength band thereof is wider than the resonance wavelength interval illustrated in FIG. 1(a) (it is desirable that a half-value width of the wavelength be narrower than two periods of the ITU grid filter 26 illustrated in FIG. 1(c)). In addition, accuracy and stability of the operating wavelength of the wavelength tunable filter 24 are not sufficient.
The ITU grid filter 26 is a wavelength selective filter for permitting a plurality of wavelengths (ITU grid wavelengths) separated from each other recommended by the International Telecommunication Union (ITU) to pass through. As illustrated in FIG. 1(c), each transmission wavelength band of the ITU grid filter 26 corresponds to each of the plurality of ITU grid wavelengths distributed in a wide band, and is equal to the resonance wavelength interval illustrated in FIG. 1(a) or smaller. Note that an interval between the ITU grid wavelengths is approximately 100 pm, approximately 200 pm, approximately 400 pm or 800 pm.
The transmission wavelength band of the wavelength tunable filter 24 is wider than the resonance wavelength interval as described above, and hence it is difficult to select a single resonance wavelength as the oscillation wavelength stably by only the wavelength tunable filter 24. Therefore, the resonator 20 selects a single resonance wavelength among the plurality of resonance wavelengths by combining the light transmission characteristics of the wavelength tunable filter 24 with the light transmission characteristics of the ITU grid filter 26. In other words, a wavelength setting signal to be applied to the wavelength tunable filter 24 is adjusted so that the peak transmission wavelength of the wavelength tunable filter 24 corresponds to the ITU grid wavelength to be made to pass through, so as to constitute a narrow-band wavelength selective filter having a desired ITU grid wavelength as the peak transmission wavelength. The wavelength tunable laser module 90 having such the structure described above can select the oscillation wavelength in a variable manner by changing the wavelength setting signal to be applied to the wavelength tunable filter 24. Note that “Recent Progress on The Wide-Band Wavelength Tunable Lasers and Modules”, Koji Kudo and other nine persons, IEICE Technical Report OPE, 2005, 25-30 (August, 2005) discloses a structure in which the first wavelength selective filter is combined with the second wavelength selective filter having the peak transmission wavelength as the ITU grid wavelength.
However, even if the narrow-band wavelength selective filter (wavelength tunable filter 24 and ITU grid filter 26) having the peak transmission wavelength as a desired ITU grid wavelength is configured, if the oscillation wavelength deviates from the peak transmission wavelength even by a little, the oscillation wavelength of the wavelength tunable laser module 90 will include an error so that light intensity is decreased.
Therefore, the wavelength tunable laser module 90 performs feedback control in which a deviation of the wavelength is detected and is corrected so that an operating oscillation wavelength agrees with the desired ITU grid wavelength. Specifically, the oscillation wavelength is controlled to agree with the desired ITU grid wavelength by a “peak search” in which light intensity of the laser beam emitted from the resonator 20 is monitored, and the oscillation wavelength is controlled so that the light intensity thereof becomes the maximum.
More specifically, the wavelength tunable laser module 90 performs “dither control” in which light intensity of a partial laser beam branched by the beam splitter 38 from the laser beam emitted from the resonator 20 is monitored by the optical output detector 42, and the phase adjustment signal to be applied to the phase adjustment region 32 of the semiconductor light amplifier 30 is increased and decreased (is changed to be a positive value and a negative value alternately) so that the light intensity thereof becomes the maximum. As described above, the effective length of the resonator 20 is expanded or contracted in accordance with the phase adjustment signal when it changes. When the effective length of the resonator 20 is expanded or contracted, a difference between the oscillation wavelength and the desired wavelength changes correspondingly. As a result, light intensity of the laser beam emitted from the resonator 20 changes.
For instance, if the light intensity increases when the phase adjustment signal (current) is increased, the phase adjustment signal is controlled to increase further. On the contrary, if the light intensity decreases when the phase adjustment signal is increased, the phase adjustment signal is controlled to decrease. In addition, if the light intensity increases when the phase adjustment signal is decreased, the phase adjustment signal is controlled to decrease further. On the contrary, if the light intensity decreases when the phase adjustment signal is decreased, the phase adjustment signal is controlled to increase. In this way, the wavelength tunable laser module 90 performs the peak search in which the phase adjustment signal is increased and decreased as the dither control so that the light intensity is maximized.
FIG. 2A is a graph illustrating an example of a relationship between the phase variation (horizontal axis) and the oscillation wavelength (vertical axis) or the light intensity (vertical axis) of the laser beam emitted from the resonator 20 (in the case where α parameter described later is zero). As illustrated in FIG. 2A, the oscillation wavelength changes substantially linearly with respect to the variation of the phase as a local aspect, but its resonator mode will be transferred to the adjacent resonator mode (mode hop) when the phase variation further changes so as to exceed a mode hop boundary. Therefore, the oscillation wavelength varies as a whole like a sawtooth with respect to a variation of the phase. In addition, the light intensity repeats its variation like a symmetric mountain shape with respect to the variation of the phase.
FIG. 2B is a graph illustrating wavelength and light intensity characteristics in which the horizontal axis and the vertical axis are respectively the oscillation wavelength and the light intensity illustrated in FIG. 2A. As illustrated in FIG. 2B, when the oscillation wavelength matches the peak transmission wavelength of the wavelength selective filter, the light intensity becomes the maximum. As a distance between the oscillation wavelength and the peak transmission wavelength increases, the light intensity is decreased. Then, if the oscillation wavelength exceeds the mode hop boundary, the number of waves in the standing wave within the resonator changes discontinuously, resulting in deterioration of the light signal. In addition, only if the oscillation wavelength becomes close to the mode hop boundary, a side mode increases in an optical spectrum or becomes multimode, resulting in deterioration of the light signal. Therefore, in order to prevent the light signal from being deteriorated, the laser device must be operated so that the oscillation wavelength is separated from the mode hop boundary. Therefore, it is necessary for the dither control in the above-mentioned peak search to change the phase adjustment signal so that the oscillation wavelength varies within the range separated from the mode hop boundary as illustrated in FIG. 2B.
As to this point, JP 8-18167 A discloses a variable wavelength light source device that changes the oscillation wavelength while keeping the same resonator mode (while preventing occurrence of the mode hop), by changing the current injected into the phase adjustment region of the semiconductor laser and the wavelength selected by a diffraction grating (wavelength selective filter) simultaneously.
In the semiconductor light amplifier 30 including the phase adjustment region 32 and the light amplifying region 34, there is the relationship as illustrated in FIGS. 3A and 3B between the variation of the refractive index (phase) and the variation of the gain (light intensity) (the ratio of those variations is referred to as “α parameter”). Note that broken lines of FIGS. 3A and 3B indicate relationships when the variation of the refractive index and the variation of the gain are not dependent on each other (in the case where α parameter is zero) similarly to FIGS. 2A and 2B.
FIG. 3A illustrates an example of a relationship between the phase variation (horizontal axis) and the oscillation wavelength (vertical axis) or the light intensity (vertical axis) of the laser beam emitted from the resonator 20. As illustrated in FIG. 3A, the oscillation wavelength does not change linearly with respect to the variation of the phase if the α parameter is not zero (solid lines). In addition, the light intensity repeats its variation like an asymmetric mountain shape with respect to the variation of the phase.
FIG. 3B is a graph illustrating wavelength and light intensity characteristics in which the horizontal axis and the vertical axis are respectively the oscillation wavelength and the light intensity illustrated in FIG. 3A. As illustrated in FIG. 3B, if the α parameter is not zero (solid line), the light intensity becomes the maximum when the oscillation wavelength matches the peak transmission wavelength of the wavelength selective filter. The variation of the light intensity becomes asymmetric with respect to the peak transmission wavelength between the short wave side (left side) and the long wave side (right side). In particular, in the range of the short wave side in which the oscillation wavelength is shorter than the peak transmission wavelength, a wavelength margin between the peak transmission wavelength and the mode hop boundary is smaller than that in the range of the long wave side. According to the experiment performed by the inventors, for instance, the wavelength margin is only 6 to 10 pm.
Further, if a light beam of narrow spectrum having light intensity of a few dBm or higher enters the optical fiber, the major portion thereof is returned to an inlet end of the optical fiber by a phenomenon called stimulated Brillouin scattering (SBS) so that electric power of the light transmitted through the optical fiber is decreased. In particular, the external resonator type laser device has a long resonator length, and hence the spectral line width of output light becomes narrow and is susceptible to the SBS.
Conventionally, in order to suppress the SBS, there is a known method in which the oscillation wavelength of the laser beam is changed in a short period, i.e., “frequency modulation” is performed on the laser beam. For instance, in order to reduce the influence of the SBS in the above-mentioned wavelength tunable laser module 90, the phase signal to be applied to the phase adjustment region 32 of the semiconductor light amplifier 30 should be modulated by an AC signal. Thus, the effective length of the resonator 20 is vibrated, which causes vibration of the oscillation wavelength of the laser beam delivered from the wavelength tunable laser module 90 (i.e., the frequency modulation is performed on the laser beam). Note that displacement of the wavelength due to the frequency modulation is approximately ±5 pm at most and that the frequency of the modulated signal is approximately 10 to 100 kHz.
However, the wavelength margin between the peak transmission wavelength and the mode hop boundary is smaller on the short wave side than that on the long wave side as for the semiconductor light amplifier 30 including the phase adjustment region 32 and the light amplifying region 34 as described above. Therefore, if the frequency modulation is further performed for suppressing the SBS on the phase adjustment signal on which the dither control is performed for the peak search, the displacement of the phase adjustment signal may reach a value that is the same as the width from the peak transmission wavelength to the mode hop boundary or larger. In other words, if the peak search and the SBS countermeasure are performed simultaneously, the side mode may increase in the optical spectrum, or the oscillation wavelength may vary discontinuously due to the mode hop, resulting in deterioration of the light signal.
Note that this problem is common to external resonator type laser devices in general, which are provided with a resonator including a wavelength selective filter and a semiconductor light amplifier having a phase adjustment region and a light amplifying region, and have an asymmetric wavelength and light intensity characteristics in which a wavelength margin between a peak transmission wavelength of the wavelength selective filter and a mode hop occurring wavelength on a short wave side is smaller than that on a long wave side.